CN105849537A - Calibration apparatus and method for computed tomography - Google Patents
Calibration apparatus and method for computed tomography Download PDFInfo
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
- A61B6/58—Testing, adjusting or calibrating thereof
- A61B6/582—Calibration
- A61B6/583—Calibration using calibration phantoms
- A61B6/584—Calibration using calibration phantoms determining position of components of the apparatus or device using images of the phantom
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
- A61B6/58—Testing, adjusting or calibrating thereof
- A61B6/582—Calibration
- A61B6/583—Calibration using calibration phantoms
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- A—HUMAN NECESSITIES
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- A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
- A61B6/03—Computed tomography [CT]
- A61B6/032—Transmission computed tomography [CT]
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- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
- G01N23/02—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material
- G01N23/04—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material and forming images of the material
- G01N23/046—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by transmitting the radiation through the material and forming images of the material using tomography, e.g. computed tomography [CT]
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Abstract
A calibration object is imaged in a radiographic system such as a CT system. The images are processed to calibrate the system, without prior measurement of the calibration object. Initial estimates are refined to improve accuracy.
Description
Technical field
The present invention relates to a kind of calibration steps, be used for using the right of calibration special (but necessarily calibration)
X-ray computed tomography (CT) system as (mirage phantom).
Background technology
X-ray imaging is used for analyzing sample as prior art, such as, for measuring or the purpose of inspection.
This sample makes the x-ray attenuation sent from radiographic source and utilizes survey device inspection detection x-ray.Through sample
The attenuation degree of the x-ray of product and the intensity of x-ray therefore detected by detector depend on sample
Feature.Such as, the part making the sample that x-ray decays by force is typically shown as on the image of detector
Dark space.
Thus, it is possible to obtain the information of sample structure, including its internal structure.
In two dimension x-ray imaging, the most only taking single sample image, it only provides ties about sample
The finite depth information of structure.
In computed tomography (CT) x-ray imaging, gather multiple two-dimensional projections shadow of sample
Picture, such as, by making sample rotate with detector relative to radiographic source.Under normal circumstances, sample is put
On a base, such as a rotating disk, and base rotates with continuous angle step around a rotary shaft.Add
The bidimensional image that work obtains is to reconstruct the three-dimensional representation of sample.
This reconstruction uses the parameter of CT system.Therefore, in order to sample is rebuild accurately, CT
The accuracy of the parameter of system is very important.Such as, use CT systematic survey sample, such as, fly
Machine parts, inside and outside time, need possible pinpoint accuracy.Equally, the parameter of system needs
There is the highest degree of accuracy.
In other words, the degree of accuracy of CT systematic parameter accuracy the highest, that rebuild is the highest.Known
By calibration determine CT system parameter (see, such as, list of references [1]-[5] and [12]).More specifically,
By this system, the sample (commonly referred to calibration object, pseudomorphism or mirage phantom) with known features is become
Picture.Analyze the image obtained, use known sample characteristic to determine the parameter of system so that it is to be used for
The imaging of other object subsequently.
Calibration object has the multiple labellings being arranged as known arrangements in auxiliary material.Under normal circumstances,
Labelling makes x-ray decay by force, and assists material to make x-ray underdamp, is so marked at x-ray
Show one's talent on image.As described above, calibration object imaging in CT system.Another kind of metering side
Method is measured, as used the coordinate measuring machine mechanical arm (CMM arm) setting up optical scanner or touch probe,
It is also used for measuring calibration object.Such as, the measured value obtained by CT system and replacement metric method, as
Distance between labelling, can be used for compareing, and is used for calibrating CT system.
The example of the method for known calibration object and use calibration object calibration CT system is recorded in ginseng
Examine document [1] to [6].
Calibration steps in list of references [1], [2], [3], [4], [5] needs before calibration, especially closes
In the 3D of labelling relative to the precise information of position, and the accuracy of calibration steps is had by any mistake
Directly affect.In list of references [6], can make great efforts to reduce but cannot eliminate the effects of the act.
List of references [12] relates to the known distance between a kind of diverse location utilizing workpiece and determines that one is
System is such as the method for amplification in CT system.Its result is limited precision.
List of references [17] relates to a kind of by adding multiple projections as determining in tomography synthesis device
The method of rotary shaft, and utilize the fact that projection of interpolation seems about axisymmetry.
The method of list of references [17] does not use the special mirage phantom comprising labelling, and be merely able to determine 2 several
What parameter.
Summary of the invention
It is an object of the invention to as solving problems of the prior art.
The aspect of the present invention is as listed by claim.
According to one aspect, the present invention provides the method for calibration radiographic systems, and this system includes
Radiographic source and/or detector, described method includes providing calibration object, makes calibration object relative to ray
Source and/or detector rotate around a rotary shaft, obtain multiple ray images of calibration object, distribute one
Represent the nominal value of the spacing of radiographic source and rotary shaft, and use described ray image and described away from
From nominal value determine the geometric parameter of system.
According to it on the other hand, the present invention provides the method for calibration radiographic systems, and this system includes
Radiographic source and/or (x-ray) detector, described method includes providing calibration object, makes calibration object
Rotate relative to radiographic source and/or detector, obtain multiple ray images of calibration object, and use
Described ray image determines the geometric parameter of system, at least without obtaining or using school in initial estimation
The prior information of quasi-object, the information i.e. obtained by the equipment in addition to the system after calibration.
In other words, the geometric parameter of system is from the image of labelling, it is not necessary to use labelling to be positioned at calibration
The precise measurements of the position in object.In order to provide dimension information, then may need extra measure with
Introduce length standard (corresponding to the nominal value of above-mentioned distance).
According to its another aspect, the present invention provides the method for calibration radiographic systems, and this system includes
Radiographic source and/or detector, described method includes providing calibration object, makes calibration object relative to ray
Source and/or detector rotate, and obtain multiple ray images of calibration object, and wherein calibration object includes many
Individual labelling, described method is included in multiple image identification labelling, uses described image capturing for often
The elliptical path of individual labelling, more preferably by the imaging track fitting ellipse to each labelling, and uses
Elliptical path is to determine the geometric parameter of system.
As mentioned below, the two-dimensional imaging track fitting ellipse at least two labelling makes the geometry of system
Parameter is determined.
More preferably, calibration object rotates around rotary shaft.Calibration object can be installed on base, such as,
One rotatable base.Base allows also to calibration object translation, such as, in three vertical direction
Translation.Or, radiographic source and/or detector can rotate around calibration object, and can also be relative to school
Quasi-object translates.
Multiple ray images correspond to the different images of x-ray difference projecting direction.In other words,
Image is to obtain in different relative rotation angle.
Embodiments of the invention come from the initial estimation to system geometric parameter, then to this initial estimation
The most perfect.This improves degree of accuracy.
Present invention also offers a suitable calibration object.
Special mirage phantom is to decline by force by arranging x-ray in the low density foam of x-ray underdamp material
Spherical labels (the such as tungsten carbide precision ball bearing) structure subtracting material forms.The ray image of mirage phantom
It is photographed substantial amounts of position and direction.Determine the 2D image coordinate of each labelling in each actinogram.
Set up Nonlinear System of Equations represent according to labelling 3D relative to these 2D images of position coordinate with
And the geometric parameter of CT system.Nonlinear iteration solver is used to obtain least square solution.All obtain
The 3D of relevant labelling relative to position or the geometric parameter about CT system or the position about mirage phantom and
/ or the accurate information in direction, can be used for improving the robustness of this solution.
In the present invention, system is calibrated need not accurate relative to position of the 3D about labelling
Prior information.Therefore, the mistake that measurement markers may cause does not introduces in calibration, such that it is able to improve
Precision.With this understanding, calibration is certainly in harmony and is solely dependent upon system and mirage phantom.Calibration also can faster,
More efficient, simpler and more cost effective, because without extra measurement equipment.Calibration can show
Field is carried out, and measures equipment or information without extra.Such as, the owner of system can calibrate system,
And without relying on manufacturer.This method can determine that 6 geometric parameters (or introduce after length standard
7 geometric parameters).
The accuracy of manufacture of mirage phantom only need to guarantee that labelling is not at ray image overlap.Without measuring mirage phantom (example
As, use contact or optical combination semiconductor lasers), and without keeping it to be dimensionally stable for a long time.
This makes the cost of mirage phantom relatively low, also makes its iconography attribute preferably be optimized.
The low density plastics or the foamed ceramics that use in the mirage phantom of x-ray CT calibration (also do not have glue
Stick) make labelling become apparent from than imaging in the prior art.
As in the prior art, if it is necessary to use contact coordinate measuring machine measurement markers, then
Support structure must more open to allow the gauge head of coordinate measuring machine to enter.Additionally, supporting construction is being sat
Must can not deflection under the active force of co-ordinate measuring machine gauge head.It addition, measured value must be (the most i.e.,
Between contact coordinate measuring machine repetition measurement) keep effectively.
All of these factors taken together can mean that general foam is not provided that enough structural stabilities, so not
It is applicable to prior art.
Equally, according to the present invention, it is not required that such as, for the adhesive of fixation mark.In calibration
During, it is important that labelling does not moves.They can be fixed by original place thus be pressed into foam.Gluing
Agent has similar attenuating (that is, being better than low density foam) with solid plastic, and therefore avoids gluing
Agent can obtain the image of more high-contrast.
In an embodiment of the present invention, calibration object can be embedded in labelling by the plastics at solid, as
Ball bearing and make.Compared with foam support structures, slightly lower contrast can be formed, but image still phase
When clear, because auxiliary material can be highly uniform.The improved method ratio of the 2D image coordinate of labelling is existing
Technology is more accurate.With it, due to method and improvement, image coordinate can navigate to sub-pixel,
Reach about 1/200thPixel.This method effect in this regard depends on that labelling can be perfectly clear
Ground imaging.
Present invention also offers a kind of device, such as, CT system and computer program or storage computer
The computer-readable recording medium of program (such as, performing method).
More preferably, radiographic systems is X-ray system, more specifically, is x-ray CT system.
Accompanying drawing explanation
After embodiments of the invention will be with reference to the explanation such as of following accompanying drawing.
Fig. 1 is the structured flowchart of CT system；
Fig. 2 is the schematic diagram of mirage phantom according to embodiments of the present invention；
Fig. 3 is the first shadowgraph figure of mirage phantom shown in Fig. 2；
Fig. 4 is the second shadowgraph figure of mirage phantom shown in Fig. 2；
Fig. 5 is the trilete rays graph of mirage phantom shown in Fig. 2；
Fig. 6 is the flow chart of steps of method according to embodiments of the present invention；
Shown in Fig. 7 is the labelling in mirage phantom；
Shown in Fig. 8 is the combination of two parts of ray images of mirage phantom in central scan；
Shown in Fig. 9 is the track of the image of two labellings in central scan；
Shown in Figure 10 is the combination of two parts of ray images of mirage phantom in shifted scanning；
Shown in Figure 11 is the track of the image of two labellings in shifted scanning；
Figure 12 is the ellipse from shifted scanning, straight line and the schematic diagram of point；
Figure 13 is straight line and another schematic diagram of point of the scanning from skew；
Figure 14 illustrates the geometric parameter of CT system；
Figure 15 illustrates a kind of method of image coordinate improving labelling；
Figure 16 is the general synoptic diagram of CT system.
Detailed description of the invention
Figure 16 illustrates the General Principle of x-ray photolithography techniques.Radiographic source S is to have directive detection
The light beam B of the centrage C of device D launches x-ray.Destination object T is arranged at radiographic source S and detection
Between device D, it is positioned in rotary shaft R.
In CT imaging, destination object T incrementally rotates around rotary shaft R.After each increment of rotation,
2 dimension projection images are obtained by detector D.Process multiple image (ray imaging projection) with structure
The 3-dimensional volume of destination object reproduces (volume diagram).
It is known to those skilled in the art from the technology of 2 dimension images structure volume diagrams.
In order to create volume diagram, need some geometric parameter of known CT system, such as the position of radiographic source S
Put, the position of detector D and rotary shaft R.It is accurately determined these parameters to need to be realized by calibration.
Fig. 1 illustrates an embodiment of CT imaging system.This system has radiographic source 2, and it is launched
The x-ray of conical shaped beam (not shown)；With detector 4, it is used for detecting what radiographic source 2 was launched
X-ray spoke.
One sample base 6 is arranged between radiographic source 2 and detector 4.Sample base 6 includes platform 8
With the rotating disk 10 on platform 8.Sample 12 is arranged on platform 8.Can by manipulator (not on platform 8
Illustrate) along beam center line (x-axis) and be perpendicular to beam center line (y and z-axis) plane in
Vertical axis converts.Rotating disk 10 is around a rotary shaft (not shown in Fig. 1, but will be described below)
Rotate.
Controller is to control computer 14 form control radiographic source, detector and sample base.Controller
14 also obtain image data from detector 4 and reconstruct volume diagram.As detailed below, controller 14 is also held
Row calibration, to determine the geometric parameter of CT system.Or, image data can be used from the transfer of CT system
Subsequent treatment in other places.
To perform calibration, as target sample is special calibration object, also referred to as mirage phantom or pseudomorphism.
Fig. 2 shows mirage phantom 16 according to embodiments of the present invention.
Mirage phantom is built as arranging multiple labelling 18 in auxiliary material 20.In order to provide high-contrast
Ray image wherein labelling is easily identified, labelling 18 decay strong for x-ray (radiopaque, or
The opaquest for x-ray).Being labeled as the spherical of precision, the 2D image of the most each labelling is sat
Mark can the highest precision determine.In an embodiment, labelling 18 is formed by tungsten carbide precision ball bearing.
18 labellings are retained in auxiliary material 20 and without using adhesive.Labelling 18 is arranged as each other
Avoid in predetermined ray image overlapping as far as possible.Preferably, labelling 18 is arranged as never in advance
Overlap in fixed ray image.Fig. 2 illustrates an example of a kind of suitable setting, but is not excluded for other
Arrange, as disclosed in list of references [3].Can be by such as, modelling or inspection design suitably set
Put.Preferably, as in figure 2 it is shown, at least one labelling near the axis of mirage phantom and at least
Another labelling is with axis relatively far apart.In other words, mark includes radially spaced labelling, relatively
Be excellently at least one labelling near axis at least one labelling near the periphery of mirage phantom.Auxiliary material
Material 20 is underdamp (ray can be saturating, or the most translucent for x-ray), so makes each labelling
Profile imaging the most with clearing.A kind of low density foam of auxiliary material 20, this low-density
Foam is mainly made up of the material of low atomic number.More preferably, auxiliary material 20 in rigidity, uniformly,
And there is low thermal coefficient of expansion.Therefore carbon or sic foam are considered as preferable.Plastic foam,
As more inadvisable in extrusion moulding type polystyrene foam, but acceptable result can be produced.Other are suitable
Material include solid plastic and pottery.As it is shown in figure 1, the x-ray CT system that mirage phantom 16 is placed on
In system, and at substantial amounts of position and direction photographs ray image.More preferably, the result bag of collection
Include complete rotation to turn around (increasing with low-angle), make rotary shaft become in the center of detector simultaneously
Picture.Central scan is important, because this is CT gathers the position generally occurred.
Fig. 3 shows the example of the ray image of the mirage phantom 16 of the Fig. 2 from central scan.
More preferably, the result of collection also includes completely revolving and turns around, and makes rotary shaft obviously face inspection simultaneously
Survey left side or the right side imaging (but whole mirage phantom 16 remains in view) of device.This shifted scanning is
Important, because being only can not institute needed for full decoupled CT system from the data of central scan
There is geometric parameter.
Preferably, detector is zero or close to zero when central scan relative to the nonconcentricity of rotary shaft,
And be noticeably greater than zero when shifted scanning, and the biggest.
More specifically, in order to concentric angle is (in the plane of detector, relative to rotary shaft " z " with vertical
D-axis " x ") " crooked ", (around rotary shaft z) " deflection ", and (" tilt " around axle x), preferably sweep in central authorities
Retouching middle deflection and be nominally equivalent to zero, in shifted scanning, deflection is non-zero.
Error in crooked, deflection, inclination is (between the value and the actual value that i.e. use in process of reconstruction
Difference) (and the error in other geometric parameter) can produce different impacts, such as part or complete
Obscuring in portion's reconstruction, or mistake scaling.
The prior art such issues that of for solving has limitation.Such as, a kind of technology assumes difference
Heart degree (especially in deflection or inclination) is close to zero, thus it is (the most right to reduce accuracy
In tilting), and cause distortion.Another kind of technology introduces serious deflection so that it is determined that tilt, but this is also
Need follow-up to carry out CT collection in identical gradient, thus increase extra complexity and make reconstruction right
Error is more sensitive.
In this technique, the deflection of central scan has close to the zero little error meaned in geometric parameter
There is minimum influence.Deflection can be introduced by translation rotary shaft sideling, and without changing x-ray source and inspection
Survey the relation between device.By making shifted scanning non-zero deflection, obscuring in calculating can be solved.
CT gather be typically deflection, crooked and tilt almost nil in the case of so that the reconstruction of object
As much as possible to deflection, crooked and tilt in error insensitive.Therefore, similarly, calibrate best
Relate to central scan at similar conditions.But, do not have additional information inclination thoroughly to obscure
(now deflection is 0) or the degree of accuracy tilted very poor (now tilting almost 0).These ambiguous can
Solved by shifted scanning.
In order to the informational linkage of central authorities and shifted scanning be got up, the invariant between both scanning is
Required.In the present embodiment, this i.e., such as, be marked in mirage phantom position and x-ray
Relation between source and detector (especially the position of principal point and radiographic source are to the distance of detector).
Fig. 4 shows the example of the ray image of the mirage phantom 16 of the Fig. 2 from shifted scanning.
More preferably, the result of collection be additionally included in different amplification (along with mirage phantom 16 towards and/or remote
From radiographic source) and at the image of different upright positions shooting.This is for providing relevant manipulator axes
Direction is important relative to the information in the direction of the pixel row and column of detector.More preferably, whole unreal
As in 16 views being included in the every time images of capture, but this is optional, need only be at rotary course
At least 2 labellings of middle holding 18 are in the view.
Fig. 5 shows the example of the ray image scanning mirage phantom 16 under high magnified glass.
Complete collection may expend considerable time (perhaps 4 hours or more), therefore exists
The geometric parameter of system or the 3D of labelling 18 relative to position it may happen that some change (such as, by heat
Change is caused) probability.Due to this reason, short period window (gather initial,
End or interstage) subset of interior repeated acquisition ray image is probably useful.This quickly scans
Allowing the long-term situation of slow scanning, this slow scanning will be determined and compensated in analysis subsequently.
This can be by than faster realizing with the track of labelling in slow scanning.
Then ray image is analyzed to determine the 2D image coordinate of labelling.
With reference to Fig. 6, calibration steps step according to embodiments of the present invention is as follows.The method comes from CT system
The first approximation of the geometric parameter of system, subsequently by performing various improvement, steps up precision.
In general, include defining unknown number (unknown parameter) according to the method for embodiment, determine redundancy
Degree of freedom, measures the deviation post of mirage phantom 16, extracts the centre coordinate of labelling, calculates initial geometry and estimate
Value, performs iterative nonlinear least-squares estimation, and residual analysis.Hereafter will be described in detail.
The analysis of the first stage according to an embodiment, loop truss method, more specifically, Canny
Rim detection and Hough loop truss method, for identifying the circle in the image about with correct diameter
(step 601).This is to identify the part likely corresponding to labelling 18 in image.This loop truss method
Multiple circles in mark image, each circle has its corresponding " mark ", and strong response or mark show
The probability that corresponding shape is circular is bigger.If mirage phantom 16 comprises all of N labelling, then lead to
Chang Di, the N response the strongest to loop truss method is selected.After determining the labelling in single image, in due order
The approximate coordinate of the labelling 18 in the image that sequence is adjacent can be predicted.Once in two or more images
After identifying labelling, the accuracy of prediction can be improved.
The 2D image coordinate (coordinate of the projection image of labelling) of labelling is improved in the analysis of next stage
(step 602).This is to complete by extracting radial intensity distribution in multiple different angles.Use
Bilinear interpolation, although other interpolation methods are likely to be suitable for.For each radial intensity distribution, really
Determine the point of greatest gradient and the Coordinate Conversion of this point is returned Descartes (Cartesian) coordinate.Use
A young waiter in a wineshop or an inn takes advantage of solution according to these fitting circles.The point of greatest gradient and the center of fitting circle and diameter are deposited
Deposit is used.
3D based on labelling 18 is relative to (approximation) prior information of position, and labelling is endowed numeral
Identifier (step 603).This can check by such as and be simply completed without measuring.For keeping away
Exempting to produce ambiguity, the setting of mirage phantom internal labeling is the most asymmetric.
Then, analyze from the 2D image coordinate of shifted scanning to determine the side of CT system and rotary shaft
To the geometric parameter with position according to a preliminary estimate, hereafter will be discussed in detail.In general, this is
By identifying that the elliptical path on each labelling tracer is (i.e., in the rotary course of calibration object
The path of the image of each labelling in multiple images is oval) and realize.According to two or many
2D image coordinate fitted ellipse (step 604) of individual labelling.According to these elliptic equations, can obtain
Straight line on detector 4, its be center slice (central slice) image (that is, be perpendicular to rotation
Rotating shaft also comprises the image of plane of x-ray source point) equation (step 605).It is likely to obtain master
The coordinate of point (be i.e. perpendicular to detector and include the image of line of x-ray source point) and x-ray source point
To the vertical dimension of detector, and then obtain the position of radiographic source S.Finally likely obtain detector
On straight line, it is the equation of image of rotary shaft.
Determine that method according to a preliminary estimate will illustrate in detail below according to present embodiment.
Fig. 7 shows mirage phantom 16, and its single marking is in coordinate-system (x, y, z) middle numbering.
Fig. 8 illustrates that in detector plane two ray images (projection) under central scan are being sat
Mark system (u, v) in example.
Fig. 9 shows the complete trajectory of the concrete labelling 18 of two under central scan.
The example of two ray images (projection) under Figure 10 display offset scanning.
The complete trajectory of two concrete labellings 18 under Figure 11 display offset scanning.
N number of labelling in image is identified to be in, and as described above, selectes 2 or multiple labelling is used for
Preresearch estimates.In the present embodiment, two labellings 18, specifically labelling (ball) 5 and mark are selected
Note (ball) 31.Identify selection marquee in multiple image captured track (corresponding x-ray
The different positions relatively that the different images of different projecting directions, i.e. calibration object rotate).In fig. 11,
Track 20 is corresponding to labelling 5, track 22 correspondence markings 31.
As shown in figure 12, the track fitting according to two labellings is oval.Figure 12 and 13 display is by ellipse
The line of equation structure and point.The ratio of Figure 13 axis is bigger than Figure 12.
Can use, such as, one of method in list of references [13] is ellipse according to above-mentioned two track fittings
Circle.Figure 12 shows and represents labelling 31 (top) and the ellipse of labelling 5 (bottom) matching.At this
In example, the equation of the ellipse 24 of ball 5 is (1.70E-06u2-4.65E-06uv+6.33E-05v2-
3.44E-04u+1.60E-02v+1=0), the equation of the ellipse 26 of labelling 31 is (1.48E-06u2-
1.56E-07uv+5.54E-05v2-2.62E-04u-1.50E-02v+1=0).
It is easy to determine the center of ellipse from their equation.In this example, labelling 5 is ellipse
The center (being represented by the x28 in Figure 12 and 13) of circle is (u=-75.75, v=-129.35), labelling
The center (being represented by the x30 in Figure 12 and 13) of 31 ellipses is (u=-81.28, v=134.92).
Can pass through, such as, list of references [14], specially Chapter 11 11.4 save, and described method makes
Two ellipses 24,26 intersect.It should be noted that be given in list of references [14] solves relevant three
The method of equation of n th order n seemingly mistake, it is possible to use another kind of method, i.e. list of references [15], specifically
It is page 183, the method be given.Oval 24,26 without real intersection point, but such as list of references [14] institute
State them and have two pairs of complex conjugate intersection points.Each complex conjugate is to connecting into a solid line.Figure 12 determines
The image of center slice is line 32.In this example, this line has equation (v=1.98E-02u+2.69).
Another line is the vanishing line 34 determined in Figure 13.In this example, this line has equation (u=
6.75E-01v+7309.63).The intersection point of two lines is the end point 36 determined in Figure 13.In this example,
End point is (u=7410.55, v=149.61).
The equation of the image according to center slice 32 and the equation of oval 24,26, it is possible to use, example
As, the method described in list of references [10] obtains the image at the center 38,40 of corresponding circle.Note
Anticipating, these are different, as shown in figure 12 with oval center.Dotted line in Figure 12 and 13 42,43,
44,45,46,48 is its ellipse being correlated with tangent line in pairwise orthogonal direction and vertical line.In described direction
One of end point 36 identical with the point determined in Figure 13.End point at other direction is rotary shaft
The picture of 50 and the joining of the picture of center slice 32.The equation of the picture of rotary shaft 50 itself can pass through
The image connecting two circle center obtains, and is identified in fig. 12.In this example, this line 50
There is equation (u=-2.09E-02v-76.58).
It is right that Figure 12 shows by the circle center 38,40 of elliptical center 28,30 and labelling 5 and 31
The straight line of the picture answered.The class liny of these lines and other labellings intersects at a single point with the picture of center slice
52.Result shows, principal point 58 is positioned at a line, and this line is by point 52 and is perpendicular to center slice
Picture.Figure 12 determining, this line is line 54, and there is equation u=-1.98E-02v+9.90.
Result is it is also shown that principal point 58 is positioned at line 56 in the same time, and this line is by end point 36 and hangs down
Straight in the picture of rotary shaft 50.Figure 13 determines this line 56.This line 56 and the line 54 determined before
Intersection point located the position of principal point 58.In this example, the line 56 through principal point 58 has equation v=
2.09E-02u-5.21, and principal point 58 is positioned at u=10.00, v=-5.00.
Additionally, list of references [16] points out the vertical dimension (i.e. focal length) from x-ray source 2 to detector 4
The square root of product equal to two distances along this line 56 of product.First distance is from principal point 58
To the distance of the picture of rotary shaft 50, second distance is to the distance of end point 36 from principal point 58.Principal point
Combination with focal length constitutes the complete description of the geometric parameter of CT system.
The side of rotary shaft can be determined by the vector product of two vectors in the plane of calculating center slice
To (as described above, center slice is perpendicular to the picture of rotary shaft the plane that comprises x-ray source point,
And it is to be found by the vector product of the vector in this plane with the vertical relation of this plane).One
Vector can (in the plane of detector) along the direction of the picture of center slice.Another vector can be penetrated from x
Line source point is to any point (in the plane of detector) on the picture of center slice.
Then, the position of rotary shaft can be determined by select rotary shaft as upper any point.
Figure 14 shows another example of the elliptical path 24,26 of two labellings along principal point 52, rotation
The picture of rotating shaft 50, and the picture of center slice 32.
As described above, above the geometric parameter of CT system and the position of rotary shaft and direction are obtained together
Complete description, but not about the 3D of labelling relative to position (the especially overall size of mirage phantom)
Accurate prior information in the case of, it is impossible to determine the vertical dimension from x-ray source to rotary shaft (i.e.
Amplification).If this distance is a nominal value of interim distribution, then can replace with substitution value
This value also can calculate the 3D of the position to rotary shaft and labelling making relative to position routinely
Suitably change.In this stage, the inexactness overwhelming majority of the 2D image coordinate of labelling is derived from insertion
Error but also note that only at spheroid as in the case of concentrating on principal point, the profile of this spheroid is
One circle.In all other circumstances, the picture of spheroid is through perspective distortion, and its profile can become ellipse
Circle.Under many circumstances, this impact is negligible, but when the picture of spheroid (such as exists very greatly
During magnification at high multiple) and during away from principal point, impact can be the biggest.Because the coordinate of principal point and x-ray source arrive
The vertical dimension of detector is known (according to the first estimation), it is possible to make up perspective distortion (step 606).
Greatest gradient point (the most storing) is changed to suitable virtual detector, greatest gradient there
Point forms a circle (rather than oval).Again store the new of this virtual detector matching to justify and it
Diameter and center.
Operating speed more slowly but more accurately algorithm improves the 2D image coordinate (step of labelling further
607).This algorithm think the narrow annulus in virtual detector be several pixel width and comprise spheroid profile (see
Figure 15).This algorithm be intended to maximization point intensity and about center mirror point intensity product (
On whole annulus) area integral.Maximum occurs when mirror point is center.It is similarly to image
Maximum cross-correlation method conventional in registration.Virtual detector builds suitable sample point, then will
It is transformed on real detector, then (such as) use secondary b-spline curve interpolation method
(quadratic b-spline interpolation) obtains intensity.(such as) simplex method can be passed through
(Nelder-Mead simplex method) or use method based on gradient maximize.Also may be used
To use additive method.
Alternatively, the 2D image coordinate to slow scan compensates with the result of coupling quickly scanning.Must
The magnitude of the compensation wanted can be used to assess heat (or the other) stability of CT system and mirage phantom, machinery
Power such as shake etc. produce change, ball bearing degree of irregularity, etc..In order to effectively utilize all can
Data, apply least square solution, it use institute markd 2D image coordinate (from skew
And central scan).Use iterative nonlinear solver minimize measurement and by the 2D of a model prediction
Difference between image coordinate (step 608).Complete model comprise the 3D of labelling relative to position,
The geometric parameter of CT system and the position of mirage phantom and direction (for each ray image).Use by
Gradually increasing free parameter carries out a series of matching.
Use from the 2D image coordinate of shifted scanning and the geometric parameter of CT system and the position of rotary shaft
Put (previously having obtained) the preliminary valuation with direction, search the 3D being used for labelling relative to the minimum of position
Two take advantage of solution (using a non-iterative solver).Then it is that complete model is asked with iterative nonlinear solver
Solving (only be shifted scanning), the limited condition of this model is: mirage phantom is in pure theory rotation (known angle
Degree step-length).
Complete the direction of the rotary shaft of central scan and the preliminary valuation of position.This can be with central and inclined
Move the difference observed in the difference of position and/or the 2D image coordinate of one or more labelling scanned
Based on (approximation) prior information being correlated with.Then use iterative nonlinear solver to improve this to estimate
Value (without attempting to solve any other part of certainly this model).
Then it is that complete model solves (simultaneously for skew and central scan) with iterative nonlinear solver,
The limited condition of this model is: mirage phantom is in two kinds of single pure theories and rotates (known angle step).
Another step optional is to relax the constraints that pure theory rotates.Mirage phantom is allowed to penetrate for each
Line image has single position and direction.Reuse iterative nonlinear solver and search least square
Solving, then it comprise beating (run-out) and swinging the measured value of (wobble) of rotary shaft.
Another optional step (step 609) is an attempt to detect the image of (and quantization) radiation imaging apparatus
The simple form of distortion.Assuming that distortion is the radial direction relative to a known center (commonly using as principal point)
Distortion, it has r'=r+a × r2Form, wherein a is the value being found and is the direction of distortion
Measured value with magnitude.Some test value correction 2D image coordinate to a, and recalculate all
Initial valuation and nonlinear least-square solution subsequently.The measured value of 2D image coordinate and the prediction of model
The summation of the variance between value is the measured value of the correctness of the test value of a and seeks its minima.Change sentence
Talk about, with the different magnitude repeated execution of steps 604 to 608 of distortion compensation, select optimum.Can
To use other to be used for the model of image distortion, such as non-radial or third-order equation model.
As it was noted above, the method up to the present introduced cannot determine that x-ray source is to rotary shaft
Vertical dimension.Correspondingly, it cannot determine the size of population of mirage phantom.This is a serious restriction,
CT system is a kind of metering CT system and needs absolute distance measurements accurately.This is accomplished by drawing
Enter length standard (step 610).It is long that method described here is easily adapted to include as described below two kind in
Arbitrary in the possible type that scale is accurate.
First kind length standard is the accurately known distance in mirage phantom between labelling.It can be a pair
Distance between labelling or the meansigma methods of this distance two or more.It is base in view of least square solution
In the nominal value of the vertical dimension of x-ray source to rotary shaft, calculate in fully equivalent solution, i.e. model unreal
As the distance between internal labeling is equal to known distance, it is inappreciable.
Equations of The Second Kind length standard is the accurately known distance between the position of mirage phantom.Such as, gathered
Journey can include that the complete of mirage phantom rotates (meanwhile, rotary shaft is in the center imaging of detector), therebetween from
X-ray source decreases known distance to the vertical dimension of rotary shaft.Need extra matching to determine at height
The direction of the rotary shaft under scanning again and position.Again, it is contemplated that least square solution, calculate completely etc.
Effect solves, and i.e. in model, distance between the position of mirage phantom, equal to known distance, is inappreciable.
This is scanned, it is possible to use bigger angle increment.Gatherer process may also include the most several scanning,
Distance of increment between the most all scan positions is known.
Known method can be used to determine the distance of above-mentioned length standard.
In camera field, the details about said method technology are recorded in list of references [7], [8], [9], [10]
[11] in.
Implement the present invention and can use the computer of suitable programming, such as, have be adapted for carrying out each
The microprocessor of the software module of function, or use special hardware module to perform each function, or
The combination of software and hardware.
Can calibrate at CT system itself, maybe data can be downloaded and be transferred to another
System is processed.Then, in subsequent treatment, that is to say, when other objects of imaging,
Calibration data and CT system are used in combination.
The aspect of the present invention and feature list as follows.
One aspect of the present invention provides a kind of method calibrating radiographic systems, and this system includes penetrating
Line source and detector, described method include provide calibration object, make calibration object relative to radiographic source and/
Or detector pivots, gather multiple ray images of calibration object, distribute one and represent radiographic source
And the nominal value (nominal range value) of the distance between rotary shaft, and by using described ray image
With the geometric parameter that nominal range value determines system.
Another aspect of the present invention provides a kind of method calibrating radiographic systems, and this system includes penetrating
Line source and detector, described method include provide calibration object, make calibration object relative to radiographic source and/
Or detector rotates, obtain multiple ray images of calibration object, and use described ray image true
Determine the geometric parameter of system, it is not necessary to about calibration object measured value information (such as prior information, i.e.
Prior to determining parameter).In other words, this method, by the measurement before or after scanning, uses non-school
Accurate calibration object, i.e. use or need the intrinsic gauging value of this object in the parameter determine system
Unknown.In the case of calibration object includes labelling, as be shown in the examples, unregulated calibration is right
The ready position liking wherein labelling is unknown.
In other words, this method can calibrate system without the information of calibration object, i.e. determines and is
The geometric parameter of system.Especially, it is not necessary to by measuring the measured value of the calibration object that calibration object obtains
Determining geometric parameter, coordinate measuring machine (CMM) can be directly passed through in this measurement, or indirectly by one
Calibrated system is carried out.This means that calibration object has more selection, because it sets without being shot
It is calculated as being suitable for CMM to measure, or there is more motility because without certain types of calibration object,
Without formerly measuring, it is not necessary to particular measurement equipment, such as CMM or other calibration systems.
As described above, according to the present invention, determine that x-ray source is to the vertical dimension of rotary shaft or length
Standard needs measured value.In other words, it is not necessary to the measurement of calibration object can obtain all several of system
What parameter, except this distance measure, it is the most required in calibration system.
Preferably, calibration object includes that multiple labelling, described method are included in ray image identification mark
The position of note.
Preferably, the method includes that by using multiple ray images be each labelling derivation elliptical path,
More preferably by the imaging track fitting ellipse to each labelling.
Another aspect provides a kind of method calibrating shooting image system, this system includes
Radiographic source and detector, described method includes providing the object being equipped with at least one labelling, makes object be somebody's turn to do
Rotate relative to radiographic source and/or detector, obtain multiple ray images of this object, described method bag
Include the picture determining at least one labelling position in multiple images, use described image to derive this at least
The path of one labelling, and use the geometric parameter of this pathway de-termination system.
It is yet another aspect of the present invention to provide a kind of method calibrating shooting image system, this system includes
Radiographic source and detector, described method includes providing a calibration object, makes calibration object relative to ray
Source and/or detector rotate, and obtain multiple ray images of calibration object, and wherein calibration object includes many
Individual labelling, described method is included in multiple image and determines multiple labelling, and it is each for using described image
Labelling derivation elliptical path, more preferably by the imaging track fitting ellipse to each labelling, and makes
With elliptical path to determine the geometric parameter of system.
Preferably, the method includes using elliptic equation, and/or based on or include making the oval side intersected
Method determines the geometric parameter of system.
Preferably, the method includes using the above-mentioned labelling in described ray image thus obtains system
Initial geometry valuation.
Preferably, the method include use circle detection method to the multiple circles identifying in each image,
And select N number of or less circle, the quantity of labelling during wherein N is calibration object.
Preferably, during geometric parameter includes the image of rotary shaft, the image of center slice and principal point one
Or it is multiple.
Preferably, the method also include obtaining distance from radiographic source to detector, the skew of object,
One or more in rotary shaft.
Preferably, the method includes compensating perspective distortion, such as, by being arrived by the image projecting of labelling
In virtual detector, wherein the image in virtual detector is more round.
Preferably, the method includes the coordinate using the image (justifying) of maximum cross-correlation method improvement labelling.
Preferably, described method includes the prediction model of derivation calibration object, and improves described the most several
What valuation and prediction model of calibration object, such as, uses a kind of iteration non-linear technology.Preferably,
The method includes analyzing and compensating residual error, such as image distortion.
Preferably, the method includes the value determining the distance represented between radiographic source and rotary shaft.
Preferably, by using the measured value of the relative position of the measured value of calibration object or calibration object
Determine this distance value.
Preferably, the method include arranging calibration object be in the rotary shaft of calibration object about with light beam
The center that center line intersects, wherein obtains calibration object and is in multiple ray shadows of described center
Picture.
Preferably, the method includes arranging calibration object and is in the rotary shaft of calibration object and deviates from light beam
The deviation post of center line, wherein obtains calibration object and is in multiple ray images of described deviation post.
Another aspect of the present invention provides the calibration object for calibrating radiographic systems, and it includes
Multiple spherical labels making x-ray decay by force is set in the material making x-ray underdamp, thus obtains
Must have the tag images of high-contrast and pinpoint accuracy.
Preferably, auxiliary material is low density foam, such as carbon or sic foam or plastic foam, example
Such as extrusion moulding type polystyrene foam or solid plastic or pottery.
Preferably, the ball bearing that labelling is made up of tungsten carbide, steel or gold.
Preferably, the arrangement of labelling is asymmetric, so will not be overlapping in ray image.
Preferably, labelling is not fixed by adhesive.
Another aspect of the present invention provides such as the described above or below side using above-mentioned calibration object
Method.
Preferably, the method include being scanned at different rates for, such as, compensate environment bar
The change of part, such as temperature.Preferably, the method includes specifying and represents between radiographic source and rotary shaft
The nominal value of distance, and use ray image and this distance nominal value to determine the geometric parameter of system.
Preferably, the method also include use calibration object (as described above) measured value obtain away from
A numerical value from nominal value.
Another aspect of the present invention provides a kind of measuring method, makes the ray calibrated in aforementioned manners
Camera chain measures object.
Another aspect of the present invention provides a kind of method calibrating radiographic systems, utilizes a calibration
Object, lacking, the described calibration obtained by the measurement equipment in addition to this radiographic systems is right
In the case of the complete reproduction of elephant, calibrate described radiographic systems.
Another aspect of the present invention provides a kind of device, it equipment including performing method as described above.
Another aspect of the present invention provides computer program or computer-readable recording medium, and it includes
For performing the computer executable instructions of said method.
List of references(being incorporated by reference into the application):
[1]EP1760457A2
[2]DE102010050949A1
[3]US20050094771A1
[4]US5442674A
[5]US7147373B2
[6]"Estimation of CT cone-beam geometry using a novel method
insensitive to phantom fabrication inaccuracy:Implications for isocenter
localization accuracy",J.Chetley Ford,Dandan Zheng,and Jeffrey F.
Williamson,Med.Phys.38,2829-2840(2011).
[7]"Camera Calibration from the Quasi-affine Invariance of Two Parallel
Circles",Yihong Wu,Haijiang Zhu,Zhanyi Hu,Fuchao Wu,ECCV 2004,
LNCS 3021,pp.190-202(2004)
[8]"Euclidean Structure from N≥2 Parallel Circles:Theory and
Algorithms",Pierre Gurdjos,Peter Sturm,and Yihong Wu,ECCV 2006,Part I,
LNCS 3951,pp.238-252(2006)
[9]"Recovering the Geometry of Single Axis Motions by Conic Fitting",
Guang Jiang,Hung-tat Tsui,Long Quan,and Shang-qian Liu,CVPR 2001,
ISBN 0-7695-1272-0/01(2001)
[10]"Single Axis Geometry by Fitting Conies",Guang Jiang,Hung-tat Tsui,
Long Quan,Andrew Zisserman,ECCV 2002,LNCS 2350,pp.537-550(2002)
[11]"Epipolar Geometry from Profiles under Circular Motion",Paulo R.S.
Mendonca,Kwan-Yee K.Wong,Roberto Cipolla(2001)
[12]DE102008044437A1
[13]"A note on the least squares fitting of ellipses",Paul L.Rosin(1992)
[14]"Perspectives on Projective Geometry",Jurgen Richter-Gebert(2011)
[15]"Numerical Recipes in C",William H.Press et al.(1992)
[16]"Camera Calibration from Surfaces of Revolution",Kwan-Yee K.
Wong et al.(2002)
[17]JP 4537090 B2
Claims (32)
1. calibration includes the method for ray imaging system for radiographic source and detector, and the method includes:
One calibration object is provided,
Described calibration object is made to rotate relative to described radiographic source and/or described detector,
Obtaining multiple ray images of described calibration object, wherein, described calibration object includes multiple mark
Note,
Multiple labelling is identified in multiple images,
Described image is used to obtain the elliptical path corresponding to each labelling, more preferably by each mark
The imaging track fitting of note is oval, and,
Described elliptical path is used to determine the geometric parameter of described system.
Method the most according to claim 1, the method wherein determining the geometric parameter of described system
Without measuring described calibration object.
3. calibration includes the method for ray imaging system for radiographic source and detector, and the method includes:
One calibration object is provided,
Described calibration object is made to rotate relative to described radiographic source and/or described detector,
Obtain multiple ray images of described calibration object, and,
Described ray image is used to determine the geometric parameter of described system, it is not necessary to the survey of described calibration object
Amount information.
Described method the most according to claim 3, wherein said calibration object includes multiple labelling,
The method includes: identifies the position of described labelling in described ray image, more preferably includes: use
Multiple ray images obtain the elliptical path corresponding to each labelling, more preferably by each labelling
Imaging track fitting is oval.
5. according to the method described in claim 1,2 or 4, including make described ellipse intersect thus
Determine the geometric parameter of described system.
6. according to the method described in aforementioned any claim, including the institute used in described ray image
State labelling and obtain the initial geometry valuation of described system.
7., according to the method described in aforementioned any claim, identify each including using loop truss method
Multiple circles in image, and select N or less than N number of described circle, wherein N is described calibration
The number of the labelling in object.
8., according to the method described in aforementioned any claim, wherein said geometric parameter includes rotary shaft
Image, one or more in the image of center slice and principal point.
Method the most according to claim 7, including obtain from described radiographic source to detector away from
One or more in, the skew of object, rotary shaft.
10. according to described method arbitrary in claim 3-9, including compensating perspective distortion, such as,
By being projeced in virtual detector by the image of labelling, wherein the image in virtual detector is more round.
11., according to described method arbitrary in claim 3-10, change including using maximum cross-correlation method
Enter the coordinate of the image (circle) of described labelling.
12. according to the method described in aforementioned any claim, including obtaining estimating of described calibration object
Model；And
Improve the described prediction model of described initial geometry valuation and described calibration object, such as, pass through
Use iterative nonlinear technology.
13. according to the method described in aforementioned any claim, including analyzing and compensating residual error, such as shadow
Image distortion.
14. according to the method described in aforementioned any claim, including determining that represents a described radiographic source
And the value of the distance between described rotary shaft.
15. methods according to claim 13, wherein said distance value is by using calibration right
The measured value of the measured value of elephant or the relative position of calibration object determines.
16., according to the method described in aforementioned any claim, are positioned at institute including arranging described calibration object
State the center that the described rotary shaft of calibration object about intersects with light beam center line, wherein when described school
Quasi-object obtains multiple described ray image when being positioned at described center.
17., according to the method described in aforementioned any claim, are positioned at institute including arranging described calibration object
The described rotary shaft stating calibration object deviates from the deviation post of described light beam center line, wherein when described school
Quasi-object obtains multiple described ray image when being in described deviation post.
18., for calibrating the calibration object of radiographic systems, are included in the auxiliary making x-ray underdamp
Multiple spherical labels making x-ray decay by force is set in material, is derived from that there is high-contrast and height
The tag images of degree of accuracy.
19. calibration object according to claim 17, wherein auxiliary material is low density foam,
Such as carbon or sic foam or plastic foam, such as extrusion moulding type polystyrene foam or solid plastic or pottery
Porcelain.
20. according to the calibration object described in claim 18 or 19, wherein said labelling be by tungsten carbide,
The ball bearing that steel or gold are made.
21. according to described calibration object arbitrary in claim 18-20, the arrangement of wherein said labelling
Being asymmetric, the most described labelling will not be overlapping in ray image.
22. according to described calibration object arbitrary in claim 18-21, the most not solid by adhesive
Fixed described labelling.
23., according to the arbitrary described method in claim 1-17, wherein use according to claim
Arbitrary described calibration object in 18-22.
24. according to the arbitrary described method in claim 1-17 and 23, including at different rates
It is scanned, such as, to compensate the change of environmental condition, such as temperature.
25., according to the arbitrary described method in claim 1-17 and 23-24, represent including specifying
The nominal value of the distance between described radiographic source and described rotary shaft, and use described ray image and
The nominal value of described distance determines the geometric parameter of described system.
26. methods according to claim 25, also include the measured value using described calibration object
Obtain a numerical value of the nominal value of described distance.
27. according to the arbitrary described method in claim 1-17 and 23-26, including using one not
The calibration object of calibration.
28. 1 kinds of calibrations include the method for the ray imaging system of radiographic source and detector, including:
The one markd object of configuration is provided,
Described object is made to rotate relative to described radiographic source and/or described detector,
Obtain multiple ray images of described object,
The position of the image of described labelling is identified in multiple images,
Described image is used to obtain the path corresponding to described labelling, and,
Described path is used to determine the geometric parameter of described system.
29. 1 kinds of measuring methods, use according to described side arbitrary in claim 1-17 and 23-28
The radiographic systems of method calibration measures an object.
30. 1 kinds of methods calibrating radiographic systems, utilize a calibration object, are lacking by except being somebody's turn to do
In the case of the complete reproduction of the described calibration object that the measurement equipment beyond radiographic systems is obtained,
Calibrate described radiographic systems.
31. 1 kinds of devices, including the method performed as described in arbitrary in claim 1-17 and 23-29
Equipment.
32. computer-readable recording mediums, including in performing such as claim 1-17 and 23-29
The computer executable instructions of arbitrary described method.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB1321003.4 | 2013-11-28 | ||
| GB1321003.4A GB2520711B (en) | 2013-11-28 | 2013-11-28 | Calibration apparatus and method for computed tomography |
| PCT/EP2014/075568 WO2015078874A1 (en) | 2013-11-28 | 2014-11-25 | Calibration apparatus and method for computed tomography |
Publications (2)
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| Publication number | Publication date |
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| JP2016538552A (en) | 2016-12-08 |
| US10478147B2 (en) | 2019-11-19 |
| CN105849537B (en) | 2021-01-12 |
| GB201321003D0 (en) | 2014-01-15 |
| JP6735667B2 (en) | 2020-08-05 |
| US20170020481A1 (en) | 2017-01-26 |
| WO2015078874A1 (en) | 2015-06-04 |
| EP3074761A1 (en) | 2016-10-05 |
| GB2520711A (en) | 2015-06-03 |
| EP3074761B1 (en) | 2018-11-21 |
| GB2520711B (en) | 2018-06-20 |
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